The present invention relates to a method of making a material alloy for an iron-based rare earth magnet, for use in, for example, motors and actuators of various types.
Recently, it has become more and more necessary to further improve the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot achieve the high remanence Br of 0.5 T or more.
An Smxe2x80x94Co type magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet that achieves the high remanence Br of 0.5 T or more. However, the Smxe2x80x94Co type magnet is expensive, because Sm and Co are both expensive materials.
Examples of other high-remanence magnets include an Ndxe2x80x94Fexe2x80x94B type sintered magnet produced by a powder metallurgical process and an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet produced by a melt quenching process. An Ndxe2x80x94Fexe2x80x94B type sintered magnet is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance. The Ndxe2x80x94Fexe2x80x94B type sintered magnet is mainly composed of relatively inexpensive Fe (typically at about 60 wt % to about 70 wt % of the total weight), and is much less expensive than the Smxe2x80x94Co type magnet. Nevertheless, it is still expensive to produce the Ndxe2x80x94Fexe2x80x94B type magnet. This is partly because huge equipment and a great number of manufacturing and processing steps are required to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for about 10 at % to about 15 at % of the magnet. Also, a sintered compact should be further processed. Furthermore, a powder metallurgical process normally requires a relatively large number of manufacturing and processing steps by its nature.
Compared to an Ndxe2x80x94Fexe2x80x94B type sintered magnet formed by a powder metallurgical process, an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet can be produced at a lower cost by a melt quenching process. This is because an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet can be produced through relatively simple process steps of melting, melt quenching and heat treating. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
As for an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1998, pp. 669-670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd3.8Fe77.2B19, where the subscripts are indicated in atomic percentages); and an Fe3B phase as its main phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe3B and hard magnetic Nd2Fe14B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are distributed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a xe2x80x9cnanocomposite magnetxe2x80x9d.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, PCT International Publication No. WO 003/03403 and W. C. Chan et. al., xe2x80x9cThe Effects of Refractory Metals on the Magnetic Properties of xcex1-Fe/R2Fe14B-type Nanocompositesxe2x80x9d, IEEE Trans. Magn. No.5, INTERMAG. 99, Kyongiu, Korea, pp. 3265-3267, 1999.
However, in producing a nanocomposite magnet by a melt quenching process, the microcrystalline structure of a rapidly solidified alloy is seriously affected by how a melt of a material alloy contact the surface of a chill roller, and the resultant magnet properties may sometimes deteriorate. Particularly when a nanocomposite magnet was be made by a strip casting process, the present inventors found it very difficult to obtain a rapidly solidified alloy having the desired micro crystalline structure uniformly and with good reproducibility. Specifically, when a melt of the material alloy was fed onto a chill roller by way of some guide such as a shoot, an oxide film was easily formed on the surface of the melt on the shoot. In that case, the melt flow was obstructed by the oxide film, and the rapid cooling process could not be carried out uniformly enough.
In order to overcome the problems described above, preferred embodiments of the present invention provide (1) a method of making an iron-based rare earth magnet material alloy that has a uniform microcrystalline structure required for a high-performance nanocomposite magnet by performing the manufacturing processing step of rapidly cooling and solidifying a molten alloy using a chill roller, constantly and with good reproducibility, and (2) a method for producing a permanent magnet by using the iron-based rare earth magnet material alloy.
According to a preferred embodiment of the present invention, a method of making a material alloy for an iron-based rare earth magnet includes the step of preparing a melt of an iron-based rare earth material alloy having a composition represented by the general formula (Fe1-mTm)100-x-y-zQxRyMz. In this formula, T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is at least one element selected from the group consisting of Y (yttrium) and the rare earth elements; and M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb. The mole fractions x, y, z and m satisfy the inequalities of: 10 at %xe2x89xa6xxe2x89xa630 at %; 2 at %xe2x89xa6y less than 10 at %; 0 at %xe2x89xa6zxe2x89xa610 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively. The method further includes the steps of feeding the melt of the material alloy onto a guide and forming a flow of the melt on the guide for tranfer to a chill roller so as to move the melt onto a region where the melt comes into contact with the chill roller; and rapidly cooling the melt using the chill roller to make a rapidly solidified alloy. The method further includes the step of controlling an oxygen concentration of the melt yet to be fed onto the guide such that the oxygen concentration is about 3,000 ppm or less in mass percentage.
In one preferred embodiment of the present invention, the method further includes the step of controlling a kinematic viscosity of the melt yet to be fed onto the guide such that the kinematic viscosity is about 5xc3x9710xe2x88x926 m2/sec or less.
In another preferred embodiment of the present invention, the rapid cooling step includes the step of using, as the guide, a shoot that controls the flow of at least a portion of the melt running down toward the surface of the chill roller rotating to bring the melt into contact with the surface of the chill roller such that the melt has a predetermined width in an axial direction of the chill roller. The shoot is disposed near the chill roller and includes a melt drain that has the predetermined width in the axial direction of the chill roller. The rapid cooling step further includes the step of making the rapidly solidified alloy from the melt that has come into contact with the chill roller.
In still another preferred embodiment, the rapid cooling step preferably includes the step of precipitating an R2Fe14B phase.
Another preferred embodiment of the present invention provides a method for producing a permanent magnet that includes the step of heat-treating the rapidly solidified alloy, prepared by the method according to any of the preferred embodiments of the present invention described above, to form a structure in which three or more crystalline phases, including at least R2Fe14B, xcex1-Fe and boride phases, are present, an average crystal grain size of the R2Fe14B phase is between about 20 nm and about 150 nm, and an average crystal grain size of the xcex1-Fe and boride phases is preferably between about 1 nm and about 50 nm.
In one preferred embodiment of the present invention, the heat-treating step includes the step of maintaining the rapidly solidified alloy at a temperature of about 550xc2x0 C. to about 850xc2x0 C. for approximately 30 seconds or more.
In another preferred embodiment of the present invention, the boride phase includes an iron-based boride phase with ferromagnetic properties.
In this preferred embodiment, the iron-based boride phase includes at least one of Fe3B and Fe23B6.
Still another preferred embodiment of the present invention provides a method for producing a permanent magnet by using a powder of the rapidly solidified alloy that has been prepared by the method according to any of the preferred embodiments of the present invention described above.
According to yet another preferred embodiment of the present invention, a method for producing a bonded magnet includes the steps of preparing a powder of the permanent magnet according to any of the methods described above, and processing the powder of the permanent magnet into the bonded magnet.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.